(Scotch Thistle). A stately architectural plant with attitude ! The large Acanthus-like leaves are densely woolly silver and tipped with real thistle barbs. At first forming an impressive evergreen rosette, during the second year, in April it mounts its advance skywards. A very stout, silver ‘trunk’ is winged, well clothed in woolly silver leaves and terminates in a broad candelabra bearing a succession of the big mauvey-pink and nectar-rich silver thistles that are depicted on tins of shortbread and which are beloved by the bees and butterflies. Usually monocarpic, this impressive plant wants lots of room and sun but is unbeatable to add structure to the border. usually self seeds very modestly so you can relocate the offspring to appropriate positions whilst small; although I have moved enormous plants in high Summer for safety’s sake (they were very prickly by the path) and gotten away with it. Planted young and well fed they can top 2.5m tall.
The term thistle can be specifically applied to the Cynareae (synonym: Cardueae) tribe within the Asteraceae or more generally to a wider group of spine bearing plants. The spines they all bear are a protection against browsing herbivores.
It is the flower structure of the true thistles that I thought might be worth delving into a little deeper. Like all of the Asteraceae, the ‘flower’ is actually a compound structure built from a large number of individual flowers. All of the flowers originate from a flat plat, the capitula, and are all of a similar size and structure. In most plants, it is the petals that provide the majority of the colour that we see, but in the thistles, the colour comes from a combination of petals and a coloured style – the female part of the flower. The overall flower is very long and narrow and divided into zones. At the base are feathery plumes (the pappus ) that are entirely encased in the bulbous urn-like involucre. These go on to develop into the thistle-down that carries the ripe seeds on the wind. Above this the flower tube colours and then divides into thread-like petals that protrude out to form a lower level to the flower tuft. Out of the end of the flower tube protrudes an elongated coloured style which is the majority of what we see. The nectaries sit just at the bottom of the pincushion flower so that visiting insects have to nose right in to get nectar collecting pollen which then gets brushed off on the styles of the next flower they visit.
Next time your thistles flower, take a moment to prize one open and marvel at the miracle of engineering they are, maybe you’ll even forgive them for driving a thorn into your finger as you do so.
The knapweeds show a flower form that is half way from a classic thistle to a typical daisy with a thistly centre with the addition of showy ray petals at the edge.
The composite flower head is the defining character of the Asteraceae. What appears to be a single daisy flower is in fact a flower head comprising of many many individual smaller flowers, in many cases two different forms of flower that together form the distinctive daisy structure. They make excellent flowers for insects to visit as each individual head is actually a multitude of nectar bearing flowers that the insect can collect from one after another.
There is a wealth of detail to study in the morphology of daisies along with a dictionary of names for the parts. I shan’t go into any great depth, but a little more detail might be interesting. The central part of the flower is called the disc and is comprised of a tightly packed array of more or less symmetrical flowers with quite small petals. These open from the outside inwards, with the outside being the oldest. Interestingly, if you study the arrangement of these flowers they are arranged in a series of spirals from the centre. You will be able to detect right and left handed spirals and if you take the time to count these spirals you will find that the number of left and right handed spirals will be two adjacent numbers from the Fibonacci series (This is a sequence of numbers made by by adding up the preceding two numbers in the sequence; Hence 1,1,2,3,5,8,13,21,34….)
The showy outer petals of a daisy are flowers of a different type. These are the assymmetrical ray florets. They lack the nectar of the disc florets and have the function of attracting insects (and gardeners!).
There is a third type of flower, a ligule, which is a assymetrical flower with one elongate petal, that can occur in the disc region. These can give flower heads a shaggy appearance and can be seen in plants such as Ligularia or Eupatorium.